Fermentation is a chemical change induced by a living organism or enzyme, such as bacteria or the microorganisms occurring in unicellular plants, which involves the aerobic decomposition of hydrocarbons to produce a desired product along with carbon dioxide. Oxygen for the fermentation is generally supplied by the provision of air to the fermentation broth which is contained within a fermentation vessel. Fermentation systems are used for the production of a large number of products such as antibiotics, vaccines, synthetic biopolymers, synthetic amino acids, and edible proteins.
It is important that the contents within the fermentation vessel be in motion to ensure the efficient uptake of oxygen, proper distribution of nutrients and the continuous progression of the fermentation reaction to attain a good product yield. Generally such motion or stirring is provided by a mechanical agitation system. Mechanical agitation systems are complicated, costly, and prone to breakdown especially because, as the fermentation process proceeds, product builds up within the fermentation vessel and the thickness or viscosity of the fermentation broth increases.
In response to such problems with mechanically agitated fermenters, there has been developed gas driven fermenters wherein the requisite broth motion is provided by rising bubbles of gas introduced into the fermenter vessel to provide oxygen for the fermentation. Because this gas is usually air, such gas driven fermenters are generally termed air-lifted fermenters or air-lifted bioreactors. Such devices are also termed bubbling columns. The air causes the broth to rise in one region of the vessel and flow downward in another region of the vessel to form a recirculating flow of broth within the fermentation vessel.
Large volumes of air are needed to effectively agitate a commercial size air-lifted fermenter or bubbling column which can have a height of from 50 to 100 feet or more. As the air bubbles rise within the bubbling column they coalesce to form larger bubbles. The coalescence is rapid due to the large volume of air injected into the bubbling column. The gas rise velocity increases with bubble size as well as with height within the column. This increases the lifting action and the turbulence within the vessel. The violent agitation caused by rapidly rising large volume air bubbles is termed heterogeneous flow. This heterogeneous flow not only enhances the lifting action within the vessel but also serves to improve stripping of the carbon dioxide, which is generated by the fermentation, out of the broth, which improves the fermentation rate.
While air-lifted fermenters have been very advantageous for use such as in the pharmaceutical and fine chemical industries, there can arise production problems caused by oxygen starvation of the fermentation reaction, especially with the use of more active biological agents that demand a lot of oxygen. There are practical limitations to how much more air one can add into an air-lifted bioreactor without blowing the contents entirely out of the reactor.
In order to address this production problem, the industry has employed the injection of an additional or supplemental oxygen-containing gas into the air-lifted fermenter to supply additional oxygen to the fermentation broth. In order to avoid having the supplemental gas bubbles coalesce with the air bubbles and thus negate the effect of the supplemental injection, the supplemental gas is provided into the air-lifted fermenter at a distance from the point where the air is provided. Often this supplemental gas is provided into the fermentation broth in the downflowing region of the vessel to assure that it is provided far from the rising air bubbles. However such a procedure reduces the circulation effect within the vessel because of the braking action of the supplemental gas bubbles which try to rise within the downflowing broth. While providing additional oxygen for the fermentation, this procedure reduces the lifting action and the carbon dioxide stripping action which are also needed to achieve high production.
Accordingly it is an object of this invention to provide an improved gas driven fermentation or bubbling column system wherein a supplemental gas may be used to provide additional oxygen for the fermentation while avoiding the detrimental effects experienced with conventional supplemental gas provision systems.